As well known in the art, biological tissue specimens are processed with liquids to dehydrate the tissue and the specimen may be subjected to other preparatory liquid treatment steps while the specimen is contained within one of many forms of a receptacle called a tissue cassette. After liquid processing, the biological tissue is ready for slicing into thin sections on a microtome for microscopic examination by a pathologist.
One common and inexpensive form of the tissue cassette used for liquid-processing of a biological tissue specimen is a liquid-permeable receptacle made from a suitable plastic, such as acetal, polyethylene, or polypropylene, and includes a cover member initially attached to the receptacle via a frangible hinge. The cover member is attached via the frangible hinge so that after liquid processing of the tissue specimen within the receptacle, the frangible hinge can be broken to remove the cover member. The cover member must be removed for attachment of the dehydrated tissue sample to the base of the tissue cassette so that the base, with attached tissue specimen, can be secured to a microtome for slicing of the tissue specimen. A typical example of such tissue cassettes having frangible hinged covers is described in U.S. Pat. No. 4,220,252. As set forth in U.S. Pat. No. 4,220,252, one of the problems associated with tissue cassettes including a cover member attached via a frangible hinge is that if the hinge portion becomes broken before the processing steps are finished, the cover member does not remain mated against the base member resulting in loss of specimens. The frangible hinges sometimes are easily broken off and sometimes very difficult to remove because small differences in frangible hinge thicknesses cause dramatic differences in needed cover removal forces.
One attempt to eliminate this problem is disclosed in U.S. Pat. No. 4,220,252 describing a tissue cassette having a cooperable detent and abutment means or latch mechanism adjacent the frangible hinge and a cooperable detent and abutment means on an opposite wall member of the tissue cassette. In accordance with U.S. Pat. No. 4,220,252, if the frangible hinge breaks prematurely, the latch mechanism adjacent the frangible hinge theoretically will maintain the cover member in place. Many times, the frangible hinge breaks before the cover member is closed so that the cover member and pair of latches must be manually and visually aligned with the base. Further, the safety latch adjacent to the frangible hinge, in this type of apparatus provides a relatively substantial degree of resistance to cover closing.
Another type of tissue cassette having a separate cover member is described in U.S. Pat. No. Re. 28,165, a reissue of U.S. Pat. No. 3,674,936. In accordance with No. Re. 28,165, a reusable stainless steel cover is attachable to a disposable cassette base by spring clips disposed at opposite ends of the cover member. Problems associated with this type of tissue cassette include the inconvenience of properly aligning the cover member with the base to contain a biological specimen for liquid processing and in maintaining a tight cover fit after repeated cover use. The users also find it much more convenient for a cover member to be initially attached to each base.